Virologist Dr Chris van Tulleken embarks on a global investigation into the hunt for the pathogen that could trigger the next pandemic and the cutting-edge science developed to tackle it. Known only as "Disease X", it is shrouded in uncertainty. Its origin is unknown, how it could spread is unclear, but its impact could be much more severe than Covid-19. To uncover what Disease X might be, where it could emerge and what traits it needs to spread, Chris follows the paths of past deadly viruses. He visits the ground zero of the Nipah virus in Malaysia, which inspired the film Contagion. He also heads to the front line of the ongoing bird flu outbreak in American dairy cattle in California.